1. Field of the Invention
The invention relates in general to solid deposition modeling, and in particular to a method and apparatus for dispensing a curable phase change material to form a three-dimensional object. The curable phase change material is dispensed in a flowable state that solidifies after being dispensed, and is then cured by exposure to actinic radiation.
2. Description of the Prior Art
Recently, several new technologies have been developed for the rapid creation of models, prototypes, and parts for limited run manufacturing. These new technologies are generally called Solid Freeform Fabrication techniques, and are herein referred to as “SFF”. Some SFF techniques include stereolithography, selective deposition modeling, laminated object manufacturing, selective phase area deposition, multi-phase jet solidification, ballistic particle manufacturing, fused deposition modeling, particle deposition, laser sintering, and the like. Generally in SFF techniques, complex parts are produced from a modeling material in an additive fashion as opposed to conventional fabrication techniques, which are generally subtractive in nature. For example, in most conventional fabrication techniques material is removed by machining operations or shaped in a die or mold to near net shape and then trimmed. In contrast, additive fabrication techniques incrementally add portions of a build material to targeted locations, layer by layer, in order to build a complex part. SFF technologies typically utilize a computer graphic representation of a part and a supply of a building material to fabricate the part in successive layers. SFF technologies have many advantages over conventional manufacturing methods. For instance, SFF technologies dramatically shorten the time to develop prototype parts and can produce limited numbers of parts in rapid manufacturing processes. They also eliminate the need for complex tooling and machining associated with conventional subtractive manufacturing methods, including the need to create molds for custom applications. In addition, customized objects can be directly produced from computer graphic data in SFF techniques.
Generally, in most SFF techniques, structures are formed in a layer by layer manner by solidifying or curing successive layers of a build material. For example, in stereolithography a tightly focused beam of energy, typically in the ultraviolet radiation band, is scanned across a layer of a liquid photopolymer resin to selectively cure the resin to form a structure. In Selective Deposition Modeling, herein referred to as “SDM” a phase change build material is jetted or dropped in discrete droplets, or extruded through a nozzle, to solidify on contact with a build platform or previous layer of solidified material in order to build up a three-dimensional object in a layerwise fashion. Other synonymous names for SDM used in this new industry are: solid object imaging, deposition modeling, multi-jet modeling, three-dimensional printing, thermal stereolithography, and the like. Often, a thermoplastic material having a low-melting point is used as the solid modeling material, which is delivered through a jetting system such as an extruder or print head. One type of SDM process which extrudes a thermoplastic material is described in, for example, U.S. Pat. No. 5,866,058 to Batchelder et al. One type of SDM process utilizing ink jet print heads is described in, for example, U.S. Pat. No. 5,555,176 to Menhennett et al. Some thermoplastic build materials used in SDM are available and sold under the names Thermojet® 2000 and Thermojet® 88 by 3D Systems, Inc. of Valencia, Calif. Also, some formulations for thermoplastic phase change build materials are disclosed in U.S. Pat. No. 6,132,665 to Bui et al.
SDM systems utilizing phase change materials have certain advantages over other SFF systems such as stereolithography. One significant advantage of SDM systems is that they are significantly less expensive than stereolithography systems. This is generally due to the use of relatively low cost dispensing devices employed in SDM systems such as ink jet print heads, instead of the expensive lasers and scanning components used in stereolithography systems. Another advantage of SDM is that the phase change build materials typically used are non-irritating and can be handled directly without the need for special handling procedures. In addition, since they do not utilize lasers that generate concentrated beams of radiation, the safety procedures required for working with laser are not needed. Hence SDM systems are preferable in office environments since special handling procedures and/or safety procedures are not required.
However, SDM systems have some disadvantages compared to stereolithography systems. One disadvantage is that the three-dimensional objects formed from conventional thermoplastic materials by SDM exhibit inferior mechanical properties compared to objects formed by stereolithography. This is generally due to the high wax content needed in the material to provide the necessary phase change characteristics for dispensing. Since wax has very little tensile strength, the high wax content significantly reduces the mechanical properties of the resultant objects formed. In addition, because most current materials used in SDM systems are phase change thermoplastic materials, the formed objects are temperature sensitive. For example, at somewhat elevated temperatures the objects start to become tacky or sticky, which is undesirable.
The need to develop a build material for use in SDM systems with improved mechanical properties has existed for quite some time. One of the first proposals was to develop an ultraviolet radiation (UV) curable phase change material that could be cured after being deposited from a dispensing device used in a SDM system. In theory, once the deposited material is cured by flood exposure to UV radiation, the final object would exhibit superior mechanical properties similar to those properties achieved in stereolithography using liquid photopolymers. Until recently, however, successfully dispensing a radiation curable phase change material in a SDM system has proven problematic.
One of the first suggestions of using a radiation curable build material is found in U.S. Pat. No. 5,136,515 to Helinski, wherein it is proposed to selectively dispense a UV curable build material in a SDM system. However no UV curable formulations are disclosed. Some of the first UV curable material formulations proposed for use in SDM systems are found in Appendix A of International Patent Publication No. WO 97/11837, where three reactive material compositions are provided. However, there is no discussion of these formulations in the specification. Further, U.S. Pat. No. 6,133,355 to Leyden et al., which lists the same three formulations and is related to WO 97/11837, indicates it is preferred to dispense them from the print head at a temperature between 90° C. and 140° C. Leyden further mentions that a reactive phase change build material formulation would have to comprise no less than 20% by weight of a reactive photopolymer component in order to realize the advantages of the reactive component. Leyden also mentions that the reactive phase change build material formulation should have a viscosity of between 18-25 centipoise at a dispensing temperature between about 125° to 130° C. However, there is no mention whether the formulations were successfully dispensed. These reactive formulations are also disclosed in U.S. Pat. No. 5,855,836 to Leyden et al. as well.
A main requirement for any material used in SDM is that it be dimensionally stable or solid at ambient temperatures and be molten or liquid at an elevated temperature induced by the application of heat. In both ink jet printing and SDM utilizing ink jet print heads, the dispensing temperature must be at least equal to the melting point at which the material transitions to the molten or liquid state. In ink jet printing processes phase change ink materials are dispensed in a molten or liquid state from ink jet print heads at a temperature of between about 85° C. to 150° C., as discussed, for example, in U.S. Pat. No. 5,380,769 to Titterington et al. In SDM, phase change thermoplastic materials are typically dispensed from an ink jet print head at about 130° C., as discussed, for example, in U.S. Pat. No. 6,133,355 to Leyden et al. Further, a reactive material formulation intended to be dispensed in a SDM method with an ink jet print head at a temperature preferably at about 140° C. is disclosed in U.S. Pat. No. 5,855,836 to Leyden et al. Also, a UV curable phase change formulation intended to be dispensed at 130° C. with a viscosity of between 20-25 centipoise is disclosed in International Patent Application WO 00/11092. Thus, the SDM techniques of the prior art indicate that curable phase change materials should be dispensed at temperatures of around 130° C., and in two-dimensional printing dispensing temperatures are generally between about 85° C. to 150° C.
However, thermal stability becomes a significant problem for curable phase change materials held these elevated dispensing temperatures for extended periods of time. In WO 00/11092 this thermal instability problem was initially discovered by measuring an increase in viscosity of the material when held at about 130° C. for extended periods of time.
It is believed this increase in viscosity is caused by thermal initiation of the cure process, wherein the long reactive molecules start to cross-link. This is undesirable as thermal initiation of the cure process can clog the dispensing orifices of the print head and cause the apparatus to malfunction. Attempting to address this problem, WO 00/11092 suggests keeping the material at a lower temperature (100° C.) in a holding container prior to being delivered to the print head which dispenses the material at a temperature of about 130° C. However, this still does not eliminate cross-linking, which can still occur in the ink jet print head and undesirably cause the print head to malfunction. Further, cross-linking may also occur in the holding container that can degrade the material to the point where it can no longer be properly dispensed.
The thermal initiation of cross-linking in curable phase change materials dispensed from ink jet print heads is no trivial problem. In order to obtain the desired mechanical strengths in the resultant objects, a sufficient quantity of reactive high molecular weight components, such as monomers, oligomers, multifunctional acrylates, and the like, are needed. However these components increase the viscosity of the formulated material in the flowable state, and often this increase is well beyond the viscosity range capability of the print head. Previously expedients in SDM have attempted to dispense the materials at the highest temperature possible where the viscosity would be low enough to meet the specifications of the ink jet print heads. However, this approach does not work for UV curable materials because the high dispensing temperatures can initiate the cure process, which can increase the viscosity of the material and undesirably effect dispensing. Adding low molecular weight monomers to lower the viscosity of the formulation can help; however, odor problems can arise, as these monomers tend to evaporate and condense within the machine causing contamination that can cause the machine to malfunction. Thus, including low molecular weight monomers is desirably minimized.
The cross-linking problem is further complicated since the trend in the ink jet printing industry is to achieve higher printing resolution by decreasing the size of the orifices in the print head. As orifice size decrease, the viscosity requirements for the material being dispensed must decrease. First generation phase change ink jet print heads required a viscosity of between about 18-25 centipoise at the dispensing temperature. Current specifications typically require a viscosity of about 13 and about 14 centipoise at the dispensing temperature, such as the Z850 printhead used in the Phaser® 850 printer available from Xerox Corporation of Wilsonville, Oreg. Next generation print heads may require even lower viscosity values, such as about 10 centipoise or less. As the viscosity values required for ink jet print heads continue to decrease, formulating phase change materials to meet these requirements becomes exceedingly difficult, particularly for radiation curable phase change materials.
More recent teachings of using curable materials in three-dimensional printing is provided in U.S. Pat. No. 6,259,962 to Gothait and in International Publication Number WO 01/26023. However, it is unknown whether these materials are phase change materials that solidify upon being dispensed. There is no mention of the melting point and freezing point of the materials. Further, in neither of these references is the problem of cross-linking and thermal stability mentioned, nor is there disclosure of the material formulations, viscosity values, or dispensing temperature.
Thus, there is a need to develop a method and apparatus capable of dispensing radiation curable phase change material in SDM while eliminating the thermal stability problems associated in dispensing the material. These and other difficulties of the prior art have been overcome according to the present invention.